1. Field of the Invention
This invention relates to heterocyclic compounds which emit laser light when properly stimulated.
2. Description of the Prior Art
In 1966 researchers first noted stimulated emissions from organic dye materials. Since that time, a wide array of organic dyes have been investigated and developed for use in what are now known as dye lasers, wherein an organic compound in solution replaces a gas or crystal (e.g. ruby) or the like as the active optical element. Laser dyes are so-called because they typically possess auxochromic groups such as oxygen and nitrogen and conjugated double bonds characteristic of classic organic dyes.
Laser dyes are economical, can be dissolved in optically clear solvents, and are not subject to cracks or other optical imperfections. The most interesting advantage of laser dyes, however, is their broadband emission. This permits the production of ultrashort pulses and the selection of output from one wavelength to another by simply "tuning" the laser to various wavelengths.
The broad family of laser dyes is classified for purposes of investigation into dyes of certain wavelength outputs: ultraviolet dyes, infrared dyes, etc. The present invention relates to blue-green laser dyes, which are of interest, for example, in underwater communications, as seawater is transmissive to light at about 480 nm wavelength.
Among the first laser dyes were the coumarins. Many of these dyes lase in the blue-green region but it was recognized very early that various substituents on and in the ring structure affect wavelength and output.
Chemical modifications of the basic coumarin structure also affect a dye's photostability. The tendency of laser dyes to break down after repeated optical stimulation is at present the greatest single drawback to the use of laser dyes. The problem is discussed by Schimitschek et al. in U.S. Pat. No. 3,891,569 and by Schimitschek et al. in "New Laser Dyes with Blue-Green Emission," 16 Opt. Comm. 313 (1976).
Ring substituents may have a dramatic effect on output and stability. The Optics Communications paper, supra, refers to two dyes, identified as AC1F and AC2F, which have a very similar ring structure, yet AC1F does not lase.
The inventors have found that AC2F, whose method of preparation has not heretofore been disclosed, is actually one of a family of stable, efficient blue-green lasing dyes.